The manufacture of homo- and copolymers of vinyl chloride by the polymerization in aqueous emulsion is well known, e.g. see the monograph by Kainer, "Polyvinylchlorid and Vinylchlorid-Mischpolymerisate" (Polyvinyl Chloride and Vinyl Chloride Copolymers), Springer publishers, Berlin/Heidelberg/New York, 1965, pp. 34 et seq.
In the emsulsion homo- and copolymerization of vinyl chloride, polymerization is generally not continued to a quantitative conversion of the monomers. In many cases, the polymerization is interrupted intentionally at a low conversion. In all instances, not inconsiderable amounts of residual monomers remain in the dispersions.
The removal of a large part of unreacted monomers is effected by expansion and/or evacuation, customarily in the polymerization vessel; the vinyl chloride, which is removed by suction, is recycled to the gasometer. In spite of these vinyl chloride recovery steps, the aqueous dispersion still contains about 1-2% by weight of monomer, based on the polymer. During the spray drying of these dispersions obtained from the emulsion polymerization, more than 95% by weight of the monomer content escapes into the atmosphere together with the dryer exhaust air. In this case, only residual monomer contents on the order of 1-20 p.p.m. remain in the spray-dried polyvinyl chloride. Such contents can be tolerated even with the stricter requirements now in force with respect to the physiological harmlessness of the products.
During the course of ever increasing efforts to reduce pollution, there is the problem of diminishing the heretofore considerable vinyl chloride emissions in the dryer exhaust air. For this purpose, it is necessary to extensively free the polyvinyl chloride dispersions from monomeric impurities before the drying procedure.
It has been known to free synthetic resin dispersions of readily volatile monomers by storing in large expansion tanks, e.g. see German Pat. No. 1,248,943, Column 2, lines 4-6. However, this mode of operation requires enormously expensive apparatus if it is to be effective.
The prior art has also suggested conducting dispersions made up of polymers, e.g. polyvinyl chloride, countercurrently to rising inert gases in spray adsorbers to thereby free them of readily volatile compounds, e.g. see DOS (German Unexamined Laid-Open Application) No. 2,162,860. This mode of operation also requires excessive expenditure in apparatus.
Steam distillation of polymer dispersions for the purpose of removing readily volatile proportions has also been described, e.g. see German Pat. No. 1,248,943, Column 2, lines 24-28. However, this mode of operation cannot be readily employed with polymer dispersions obtained in emulsion polymerization, since such dispersions, due to their emulsifier content, have such a low surface tension that strong foaming occurs during a steam distillation.
In a conventional, discontinuously operating process described in German Pat. No. 1,248,943, the foam formed during the steam distillation of dispersions with a surface tension of below 40 dynes per centimeter is broken up by subjecting the thus-produced steam-dispersion foam mixture to a rapid pressure drop at flow velocities of more than 100 m./sec. The broken-up foam is recycled from the separator into the steam distillation, but the process operates discontinuously.
An expressly emphasized in German Pat. No. 1,248,943, at Column 5, lines 19-22, the respective process can be operated in a continuous fashion only if several units are connected in series as a cascade. Naturally, such a series connection of several, discontinuously operating units involves a great expenditure for capital equipment. Additionally, the series connection of several units of the aforedescribed type represents a large mechanical load on the dispersions as they are exclusively manufactured in modern, large-scale technical methods which have a high solids content and are low in emulsifier. In a highly undesirable manner, deposits of sediment and coagulate are formed in the pipelines between the degasification vessels from the comparatively low-stability dispersions.
In order to avoid these difficulties, German Patent Application No. P 24 40 957 corresponding to U.S. Pat. No. 4,020,032, issued April 26, 1977 describes a process according to which the dispersion is mixed continuously with steam in a flow pipe, wherein the flow velocity of the mixture is 1-20 m./sec. and the pressure in the flow pipe is 200-800 millibars, the mixture exiting from the flow pipe at a velocity of 30-100 m./sec. and being fed to an evacuated degasification vessel wherein the pressure is by 40-270 millibars lower than the pressure in the flow pipe and being continuously withdrawn from this degasification vessel. The temperatures utilized in this process are, in correspondence with the pressure, no higher than 94.degree. C., since it had to be assumed that dispersions of polymers and copolymers of vinyl chloride cannot be exposed to temperatures higher than 95.degree. C. since according to general experience such dispersions are thermally damaged at higher temperatures, producing discoloring of the polymers as well as formation of coagulate and sediment.